Counter target spell unless its controller pays , where X is the greatest power among creatures you control.
"Like water, mana will drain from an inferior vessel." —Jalan Tosk, Simic forcemage
Official Rulings for Spell Rupture
1/24/2013 : The value of X is the greatest power among creatures you control when Spell Rupture resolves. If you control no creatures at that time, X will be 0. The target spell's controller can choose to pay (simply by indicating he or she wishes to do so). That player can also choose to not pay and the spell will be countered.